The oil and gas industry typically conducts comprehensive evaluation of underground hydrocarbon reservoirs prior to their development. Formation evaluation procedures generally involve collection of formation fluid samples for analysis of their hydrocarbon content, estimation of the formation permeability and directional uniformity, determination of the formation fluid pressure, and many others. Measurements of such parameters of the geological formation are typically performed using many devices including downhole formation testing tools.
Recent formation testing tools generally comprise an elongated tubular body divided into several modules serving predetermined functions. A typical tool may have a hydraulic power module that converts electrical into hydraulic power; a telemetry module that provides electrical and data communication between the modules and an uphole control unit; one or more probe modules collecting samples of the formation fluids; a flow control module regulating the flow of formation and other fluids in and out of the tool; and a sample collection module that may contain various size chambers for storage of the collected fluid samples. The various modules of a tool can be arranged differently depending on the specific testing application, and may further include special testing modules, such as NMR measurement equipment. In certain applications the tool may be attached to a drill bit for logging-while-drilling (LWD) or measurement-while drilling (MWD) purposes. Examples of such multifunctional modular formation testing tools are described in U.S. Pat. Nos. 5,934,374; 5,826,662; 5,741,962; 4,936,139, and 4,860,581, the contents of which are hereby incorporated by reference for all purposes.
In a typical operation, formation-testing tools operate as follows. Initially, the tool is lowered on a wireline into the borehole to a desired depth and the probes for taking samples of the formation fluids are extended into a sealing contact with the borehole wall. Formation fluid is then drawn into the tool through inlets, and the tool can perform various tests of the formation properties, as known in the art.
Prior art wireline formation testers typically rely on probe-type devices to create a hydraulic seal with the formation in order to measure pressure and take formation samples. Typically, these devices use a toroidal rubber cup-seal, which is pressed against the side of the wellbore while a probe is extended from the tester in order to extract wellbore fluid and affect a drawdown. This is illustrated schematically in FIG. 1, which shows typical components of an underground formation tester device, such as a probe with an inlet providing fluid communication to the interior of the device, fluid lines, various valves and a pump for regulating the fluid flow rates. In particular, FIG. 1 shows that the rubber seal of the probe is typically about 3-5″ in diameter, while the probe itself is only about 0.5″ to 1″ in diameter. In various testing applications prior art tools may use more than one probe, but the contact with the formation remains at a small point area.
The reliability and accuracy of measurements, made using the tool illustrated in FIG. 1, depends on a number of factors. In particular, the producibility of a hydrocarbon reservoir is known to be controlled by variations in reservoir rock permeability due to matrix heterogeneities. It is also well known that underground formations are often characterized by different types of porosity and pore size distribution, which may result in wide permeability variations over a relatively small cross-sectional area of the formation. For example, laminated or turbidite formations, which are common in sedimentary environments and deep offshore reservoirs, are characterized by multiple layers of different formations (e.g., sand, shale, hydrocarbon). These layers may or may not be aligned diagonally to the longitudinal axis of a vertical borehole and exhibit differing permeabilities and porosity distributions. Similarly, as shown in FIG. 2, in naturally fractured formations whose physical properties have been deformed or altered during their deposition and in vugular formations having erratic pore size and distribution, permeabilities to oil and gas may vary greatly due to the matrix heterogeneities.
For example, in laminated or turbidite reservoirs, a significant volume of oil in a highly permeable stratum, which may be as thin as a few centimeters, can be trapped between two adjacent formation layers, which may have very low permeabilities. Thus, a formation testing tool, which has two probes located several inches apart along the longitudinal axis of the tool with fluid inlets being only a couple of centimeters in diameter, may easily miss such a rich hydrocarbon deposit. For the same reasons, in a naturally fractured formation, in which oil or gas is trapped in the fracture, the fracture acts as a conduit allowing formation fluids to flow more freely to the borehole and causing the volume of hydrocarbon to be underestimated. On the other hand, in a vugular formation a probe may encounter an oil vug and predict high volume of hydrocarbon, but due to the lack of connectivity between vugs such high estimate of the reservoir's producibility will be erroneous.
One solution to the above limitations widely used in prior art wireline formation testers is to deploy straddle packers. Straddle packers are inflatable devices typically mounted on the outer periphery of the tool and can be placed as far as several meters apart from each other. FIG. 3 illustrates a prior art device using straddle packers (cross-hatched areas) in a typical configuration. The packers can be expanded in position by inflating them with fluid through controlled valves. When expanded, the packers isolate a section of the borehole and samples of the formation fluid from the isolated area can be drawn through one or more inlets located between the packers. These inflatable packers are used for open hole testing and have historically been deployed on drill pipe. Once the sample is taken, the straddle packers are deflated and the device can be moved to a new testing position. A number of formation tester tools, including the Modular Formation Dynamics Tester (MDT) by Schlumberger, use straddle packers in a normal operation.
Although the use of straddle packers may significantly improve the flow rate over single or dual-probe assemblies because fluid is being collected from the entire isolated area, it also has several important limitations that adversely affect its application in certain reservoir conditions. For example, it is generally a practice in the oil and gas industry to drill boreholes large enough to accommodate different types of testing, logging, and pumping equipment; therefore, a typical size of a borehole can be as much as 50 cm in diameter. Since the diameter of a typical formation-testing tool ranges from 10 cm to 15 cm and an inflated packer can increase this range approximately by an additional 10 cm, the packers may not provide sufficient isolation of the sampled zone. As a result, sufficient pressure may not be established in the zone of interest to draw fluids from the formation, and drilling mud circulating in the borehole may also be pumped into the tool.
Furthermore, while straddle packers are effective in many applications, they present operational difficulties that cannot be ignored. These include a limitation on the number of pressure tests before the straddle packers deteriorate, temperature limitations, differential pressure limitations (drawdown versus hydrostatic), and others. Another potential drawback of straddle packers includes a limited expansion ratio (i.e., out-of-round or ovalized holes).
A very important limitation of testing using straddle packers is that the testing time is invariably increased due to the need to inflate and deflate the packers. Other limitations that can be readily recognized by those of skill in the art include increased pressure stabilization—large wellbore storage factor, difficulty in testing a zone just above or just below a washout (i.e., packers would not seal); hole size limitations of the type discussed above, and others. Notably, straddle packers are also susceptible to gas permeation and/or rubber vulcanizing in the presence of certain gases.
Accordingly, there is a need to provide a downhole formation testing system that combines both the pressure-testing capabilities of dual probe assemblies and the large exposure volume of straddle packers, without the attending deficiencies associated with the prior art. To this end, it is desirable to provide a system suitable for testing, retrieval and sampling from relatively large sections of a formation along the surface of a wellbore, thereby improving, inter alia, permeability estimates in formations having heterogeneous matrices such as laminated, vugular and fractured reservoirs. Additionally, it is desired that the tool be suitable for use in any typical size boreholes, and be deployable quickly for fast measurement cycles.